There is known an image forming apparatus which transfers toner supported on an image carrier such as a photoreceptor, an intermediate transfer body or the like onto a paper in a transfer unit and which fixes the toner transferred onto the paper by thermal fixing in a fixing unit.
In FIG. 13, an outline of a transfer unit 210 and a fixing unit 220 of a conventional image forming apparatus 200 is shown.
The transfer unit 210 comprises a photoreceptor drum 211 to support toner, a transfer belt 212 and rollers 213,214 and 215. The photoreceptor drum 211 and the roller 213 are disposed so that the outer circumferences thereof face each other across the conveyance passage of a paper P. A paper P is nipped between the photoreceptor drum 211 and the transfer belt 212 at the facing part of the photoreceptor drum 211 and the roller 213 to be conveyed in the direction of the arrow. In this conveyance process, the toner image on the photoreceptor drum 211 is transferred onto the paper P.
The fixing unit 220 comprises a fixing roller 221 having a heat source, a fixing belt 222 and rollers 223, 224 and 225. The fixing roller 221 and the roller 223 are disposed so that the outer circumferences thereof face each other across the conveyance passage of the paper P. The paper P is nipped between the fixing roller 221 and the transfer belt 222 at the facing part of the fixing roller 221 and the roller 223 along to the upstream side in the conveyance direction of the paper P to be conveyed in the direction of the arrow. In this conveyance process, the toner image on the paper P is fixed by thermal fixing by the fixing roller 221.
The transfer belt 212 and the fixing belt 222 are activated by each of the rollers provided within each of the rings being driven. There is a possibility that position displacement occurs in the widthwise position of the belt with the activation of each belt. The position displacement may cause twisting, rippling and the like of the belt and may cause various types of poor conditions such as poor contact between the photoreceptor drum 211 and the paper P and between the fixing roller 221 and the paper P, drifting of image, roughness in brightness and the like. In order to prevent such poor conditions, a structure for inhibiting or correcting the position displacement is provided in the image forming apparatus. For example, there is a structure to inhibit position displacement of the belt by having guide members provided at both side portions of the widthwise position of each belt (for example, patent document 1). Alternatively, there is a structure where any one of the rollers within the ring of each belt is provided so that the angle thereof with respect to the belt can be changed and where the widthwise position of the belt can be controlled by guiding by changing the angle (for example, patent document 2).
Among the above structures, in a case of the structure where the position displacement of the widthwise position of the belt is inhibited by the guide members as in patent document 1, there is a possibility that the position displacement occurs by the belt going over the guide members when the position of the belt is greatly displaced. Therefore, when the position control of the belt is preferred to be carried out surely, the structure to carry out a control of the widthwise position of the belt by carrying out the angle control of the roller as in patent document 2 is applied.
Recently, due to the downsizing of image forming apparatus, there is an image forming apparatus where a space E between the position where a paper P is nipped by the transfer unit 210 (hereinafter, called “transfer nip”) and the position where the paper P is nipped by the fixing unit 220 (hereinafter, called “fixing nip”) is made to be smaller than the width of the paper P in the conveyance direction. In such image forming apparatus, there is a timing when the paper P is nipped by both of the transfer nip and the fixing nip. When the control of the widthwise position of each belt by the angle of the roller is carried out when the paper P is nipped by both of the transfer nip and the fixing nip, there is a possibility that distortion occurs in the paper P. Hereinafter, the distortion will be described with reference to FIG. 14.
In FIG. 14, an example of a case where distortion occurs in the paper P is shown. The paper P is nipped by the transfer unit 210 at the position of transfer nip G and is nipped by the fixing unit 220 at the position of fixing nip H. At this time, when the control of position is carried out in different directions for the transfer belt 212 and the fixing belt 212, the paper P is to be pulled in opposite directions from each other at the position of transfer nip G and at the position of fixing nip H, and distortion occurs at the portion between the transfer nip G and the fixing nip H. The distortion causes various types of poor conditions in the image forming and deterioration in quality of the image forming such as causing a poor fixation of toner and further causing wrinkles on the paper by the paper P being nipped by the fixing unit 220 in the distorted condition.
Given this fact, there is a structure to inhibit the angle control of the roller to correct the position displacement of the belt when the paper is at the fixing unit (for example, patent document 3).    Patent document 1: Japanese Patent Application Laid-Open Publication No. 3-25477    Patent document 2: Japanese Patent Application Laid-Open Publication No. 8-262903    Patent document 3: Japanese Utility Model Application Laid-Open Publication No. 5-30854